This invention relates to a light source. In particular, the invention relates to a high-power light source in the form of a light emitting diode (LED) package.
Light emitting diodes (LEDs) fabricated from silicon wafer are commonly used to generate light in a variety of applications ranging from simple low-power indication lights to higher-power LED traffic light clusters and LED matrix video displays. Typically, the light emitting diode die is assembled into a sealed package containing electrical connections between the die and terminal pads exposed on an outer surface of the package. Such a package enables simple connection of the diode to external circuitry and, due the sealing properties of the package, protects the die from external damage.
Recently, there has been a drive to make smaller surface mount LED packages which allow the LED to be reliably mounted onto a printed circuit board substrate at relatively high speeds. By making individual LED packages smaller, the number of LED dies per unit area in a multiple LED package may be increased. Furthermore, when the LED is mounted onto a circuit board, the thickness of the assembled circuit board can be reduced.
Today's surface mount LED packages are available in a wide variety of configurations. FIG. 1 shows one typical surface mount LED package 100 comprising an LED die 110 mounted on a circuit board substrate 120 with a transparent material 130 encapsulating the LED 110. The package includes a pair of conductive interconnects 140, 142 for coupling the LED to external circuitry. A first electrode on the bottom surface of the LED 110 is mounted on and electrically coupled to one of the pair of conductive interconnects 140. A very small wire 144 is then “wire bonded” or welded at one end to a second electrode on the top surface of the LED 110, and at the other end to the other one of the pair of conductive interconnects 142.
A drawback with the LED package of FIG. 1 is its inefficiency in dissipating heat away from the LED die 110. The circuit board substrate 120 and the transparent encapsulant material 130 are typically made from thermally insulating materials which “trap” heat in LED die 110. For example, the circuit board material FR4 has a thermal conductivity coefficient of 0.2-0.3 watts per meter kelvin, while the encapsulant material Able-Bond 826 has a thermal conductivity coefficient of approximately 2 watts per meter kelvin. Better heat conductivity is provided by the conductive interconnects 140, 142 which if made from copper would have a thermal conductivity coefficient of 400 watts per meter kelvin. However, the rate of heat dissipation from the conductive interconnects is severely limited by i) the small cross-sectional area of the interconnects, and ii) the relatively large distance of the heat path offered by the interconnects.
Demand for higher brightness surface mount LED packages is increasing in many fields such as in automotive and decorative lighting applications. Higher brightness can be achieved by increasing the current or power supplied to the LED die.
The poor heat dissipation properties of the LED package of FIG. 1 limit the ability of the package to operate at higher power and thus increased brightness. Without efficient heat dissipation, the increased power supplied to the package rapidly increases the temperature of the die resulting in poor light extraction efficiency and even permanent damage to the die.